Host species identity drives temporally stable octocoral microbiomes in Florida

Abstract Octocorals are foundational components of Caribbean reef ecosystems, yet their bacterial microbiomes remain poorly characterized compared to scleractinian corals. Understanding the relative contributions of host identity, morphology, and environmental factors in shaping these microbial partnerships is critical for predicting octocoral responses to environmental change. We characterized bacterial communities associated with four ecologically important Florida octocoral species over three years using 16S rRNA gene sequencing: the encrusting species Briareum asbestinum and Erythropodium caribaeorum, and the branching species Eunicea flexuosa and Muricea muricata. Host species identity emerged as the strongest predictor of microbiome composition, explaining 32.3% of bacterial community variation and substantially outweighing temporal and spatial effects. Core microbiome analysis revealed species-specific bacterial associations, with all four species harboring distinct Endozoicomonas variants as dominant symbionts. Encrusting species hosted taxonomically diverse microbiomes, while branching species maintained highly specialized communities dominated by Endozoicomonas and Mycoplasma. Bacterial communities within each host species remained stable across sites, depths, and years, with temporal and spatial factors explaining only 1.8% and 2% of variation, respectively. These findings demonstrate strong host control over microbial colonization and indicate that stable octocoral-bacteria associations may contribute to octocoral ecological resilience on western Atlantic reefs. Competing Interest Statement The authors have declared no competing interest.